docs: def install_via_requirements(requirements_str, force=False):
      """
      Download multiple Quilt data packages via quilt.xml requirements file.
      """
      if requirements_str[0] == '@':
          path = requirements_str[1:]
          if os.path.isfile(path):
              yaml_data = load_yaml(path)
              if 'packages' not in yaml_data.keys():
                  raise CommandException('Error in {filename}: missing "packages" node'.format(filename=path))
          else:
              raise CommandException("Requirements file not found: {filename}".format(filename=path))
      else:
          yaml_data = yaml.safe_load(requirements_str)
      for pkginfo in yaml_data['packages']:
          info = parse_package_extended(pkginfo)
          install(info.full_name, info.hash, info.version, info.tag, force=force)